Method and arrangement for performing drilling operations

ABSTRACT

An arrangement and a method to control and regulate the bottom hole pressure in a well during subsea drilling at deep waters: The method involves adjustment of a liquid/air interface level in a drilling riser. The arrangement comprises a drilling riser with an outlet at a depth below the water surface. The outlet is connected to a subsea pumping system with a flow return conduit back to a drilling vessel. The intention of the system is to transport the drilling fluid and the formation particles to the surface on the drilling unit prior to setting structural pipe on the seabed and when drilling at least one hole section after the first surface structural casing have been set. The apparatus is used in order to drill all surface hole sections with the riser installed in order to avoid “pump &amp; dump” procedures and to recover all mud and chemicals.

RELATED APPLICATIONS

This application is a national phase application of PCT ApplicationSerial Number PCT/NO2004/000069 filed Mar. 12, 2004, which, in turn,claims priority to Norwegian Application Ser. No. NO 2003 1168, filedMar. 13, 2003. Each of these applications is herein incorporated in itsentirety by reference.

FIELD OF THE INVENTION

The present invention relates to a particular arrangement for use whendrilling a hole in the ocean floor from an offshore structure thatfloats or is connected to the seabed by other means. More particularly,it describes a drilling riser system so arranged that the pressure inthe bottom of an underwater borehole can be controlled so that thehydrostatic pressure inside the riser is equal to or slightly below thatof seawater at that depth and not higher than the formation strength ofthe weakest section of the borehole.

BACKGROUND OF THE INVENTION

In all present drilling operations to date in offshore drilling with asemi submersible rig or drillship, this top hole drilling is performedriserless. The debris and drill cuttings are until now handled in twodifferent ways. 1) The returns are discharged and flow freely intoseawater as the drilling fluid and formation debris are pumped up thehole. The drilling fluid and formation will then be spread out on theseabed around the borehole. 2) After the well is spudded and the firststructural/conductor casing is set, some equipment is run on the drillstring that will connect to a suction hose and a pump placed on seabed.The majority of the drill fluid and cuttings is then sucked from the topof the hole and pumped away from the drill site to a different locationon seabed. This cutting transport system will not remove the cuttingsfrom the seabed but just re-locate them.

Lately concepts have been presented that will pump the return fromseabed up to the drilling platform thorough a separate hose with thehelp of a pumping system on seabed after the structural or conductorcasing has been set. This is indicated in patent NO312915. Here the pumpis place on the seabed and no drilling riser is installed.

SUMMARY OF THE INVENTION

This invention defines a particular novel arrangement, which can be usedfor drilling a subsurface hole without having to discharge subsurfaceformations to the surrounding seabed when drilling the hole prior toinstalling the surface conductor (structural) steel pipe and prior toinstalling the surface casing, at which point the riser and subsea BOPis installed in conventional drilling. By performing drilling operationswith this novel arrangement as claimed, all formation and soil will becirculated and pumped up to the surface vessel or platform. Thearrangement comprises the use of prior known art but is arranged so thatnew drilling methods can be achieved. By arranging the various systemscoupled to the drilling riser in this particular way, totally new andnever before used methods can be performed.

Referring to the figures, experience from drilling operations in uppersoil layers has shown that the subsurface formations to be drilledusually have very low fracture strength (301) close to the seabed and itis often close to that of seawater (302). This dictates that drilledformation will have to be disposed on seabed since the formationstrength is not high enough to support the hydrostatic pressure from thecombined effect of drilling mud and the suspended drilled formationsolids in a drilling riser up to the drilling platform (304). This isthe reason it is not possible to install a conventional drilling riserand take the returns to the surface, before a casing is set so deep thatit will isolate the weaker formation and that the soil strength is highenough to support a liquid column of water and formation cuttings(debris) up to the drilling unit above sea level.

The two uppermost sections of the hole are normally drilled riserless,without a drilling riser. Often this “pump and dump” procedure cause forexcessive amount of drilling mud, barite weighting materials, formationsolids and other chemicals to be dumped to the ocean. Besides thispractice being expensive it is also a wasteful process that can beharmful to marine life on the ocean floor.

In deeper waters as the hole deepens, the difference between theformation pore pressure and the formation fracture pressure remains low.The fracture gradient is so low that it can not support the hydrostaticpressure from a full column of seawater and formation cuttings up to thedrilling platform. In addition to the static hydraulic pressure actingon the formation from a standing column of fluid in the well bore thereare also the dynamic pressures created when circulating fluid throughthe drill bit. These dynamic pressures acting on the bottom of the holeare created when drill fluid is pumped through the drill bit and up theannulus between the drill string and formation. The magnitude of theseforces depends on several factors such as the rheology of the fluid, thevelocity of the fluid being pumped up the annulus, drilling speed andthe characteristics of the well bore/hole. Particularly for smallerdiameter hole sizes these additional dynamic forces can becomesignificant. Presently these forces are controlled by drillingrelatively large holes thereby keeping the annular velocity of thedrilling fluid low and by adjusting the rheology of the drilling fluid.This new pressure seen by the formation in the bottom of the hole causedby the drilling process is often referred to as Equivalent CirculatingDensity (ECD).

Since this ECD effect can be neutralized by the system as described inpatent application PCT/NO02/00317 the surface hole can be drilled deeperthan with conventional drilling methods. This is an advantage since thenext section can also be drilled deeper hence it is possible to thedrill the well with fewer casings if the surface casing can be setdeeper. Hence considerable economic effects can be expected fromdrilling the surface hole deeper.

The new method presented here will also allow for the riser to be runbefore setting any casings. The reason for this possibility is that thehydrostatic pressure at the bottom of the riser can be regulated to thesame or less than that of seawater from sea level, regardless of thefluid density inside the drilling riser. This is achieved by having anoutlet on the riser below the surface of the water that is connected toa pump system that will be able to regulate the liquid level inside thedrilling riser to a depth below sea level. In this particular way willit be possible to pump drilling fluid (mud) through the drill string andup the annulus between the riser and the drill string together withformation cuttings without fracturing or loosing returns caused by theweak topsoil formations.

Below are some aspects the present invention will be used for.

In one aspect the present invention in a particular combination givesrise to new, practically feasible and safe methods of drilling thesurface hole deeper with the riser installed from floating structures.In this aspect, benefits over the prior art are achieved. More preciselythe invention gives instructions on how to drill and control thehydraulic pressure exerted on the formation by the drilling fluid at thebottom of the hole being drilled by varying the liquid level in thedrilling riser. With this novel invention, both kick and handling ofhydrocarbon gas can be safely and effectively controlled. It is possibleto add a surface BOP on top of the drilling riser (410)

Since the pressure in the end of the riser can be defined by the densityof the liquid and the vertical height of the liquid column, the surfacestructural conductor can be run on the end of the riser and bedrilled/undereamed or jetted in place with returns being circulated tothe surface with the help of the Low Riser Return System (LRRS). Nocuttings or formation is being deposited on the seabed or to the ocean.

Once the structural conductor is jetted in place the riser isdisconnected at LRMP (233) and the telescope joint (221) removed and theriser lengthened. The riser is reconnected and the second surface holefor the surface casing can be drilled with drilling mud. All returns andmud will be circulated to surface with the LRRS. Since the bottom holepressure can be designed to stay below the fracture pressure of theformation being drilled, the surface hole can be drilled deeper.

After the structural casing is in place a surface BOP can be installedon top of the riser. The BOP will be used in case of shallow pockets ofhydrocarbons are encountered and hydrocarbons are circulated into theriser when drilling the hole for the surface casing. There may be atleast one choke line in the upper part of the drilling riser of equal orgreater pressure rating than the drilling riser. By incorporating theabove features a well functioning system will be achieved that cansafely perform drilling operations of the top two hole sections. Byhaving a surface blowout preventer on top of the drilling riser, allhydrocarbons can safely be bled off through the drilling rig's chokeline manifold system.

In one aspect the present invention overcomes many disadvantages ofother attempts and meets the present needs by providing methods andarrangements whereby the fluid-level in the riser can be dropped belowsea level and adjusted so that the hydraulic pressure in the bottom ofthe hole can be controlled by measuring and adjusting the liquid levelin the riser in accordance with the dynamic drilling processrequirements. Due to the dynamic nature of the drilling process theliquid level will not remain steady at a determined level but willconstantly be varied and adjusted by the pumping control system. Apressure control system controls the speed of the subsea mud lift pumpand actively manipulates the level in the riser so that the pressure inthe bottom of the well is controlled as required by the drillingprocess. With the methods described it is possible to regulate thepressure in the bottom of the well without changing the density of thedrilling fluid.

The ability to control pressures in the bottom of the hole and at thesame time and with the same equipment being able to contain and safelycontrol the hydrocarbon pressure on surface makes the present inventionand riser system completely new and unique.

The method of varying the fluid height can also be used to increase thebottom-hole pressure instead of increasing the mud density. This meansthat the surface hole can be drilled at an angle/different than theriser while controlling the bottom hole pressure. This is not easilyachieved with a conventional riser or achieved drilling riserless due toproblems with hole stabilities when drilling with un-weighted seawaterin an angularly deviated borehole hole.

Normally as drilling takes place deeper in the formations the porepressure will also vary. In conventional drilling operation the drillingmud density has to be adjusted. This is time-consuming and expensivesince additives have to be added and is discharged out to the seawithout being able to reclaim the mud and chemicals. With the LRRSsystem the mud will be reclaimed at surface hence a more purpose fitdrilling mud can be used which will drill a more gauged hole and bettersamples and cores can be collected.

FIG. 1 a schematic overview of the arrangement, including a depth versuspressure graph overlay.

EXPLANATION OF THE FIGURES

The (drilling) riser tube 201 has a lower outlet between the sea leveland ocean floor with valves 204 that will divert the fluid in the risertube into the submersible pump system which will pump the fluid andsolids back up to the surface.

By being able to drop the air/liquid level 210 in the riser to a levelbelow sea level, it is also possible to create a pressure inside saidriser which is below that of seawater, which can be seen from gradient305 which can be below that of gradient 302 which is seawater pressuregradient from sea level 200. This implies that seawater will flow intothe end of the riser tube up into the lower outlet of the riser tubeinto the subsea pump 202 which will pump the content through the returnconduit 220 back to a surface vessel.

When starting the drilling operation from a floating vessel the firststructural conductor 236 can be run on the end of the riser tube 201.The conductor housing 234 is connected to the surface structuralconductor and the riser connected to the conductor housing 234 with apin connector 233. The structural conductor is lowered into the seabedprior to running the drill string 211. When the drill string 211 is runinside the riser 201 down to the seafloor 300, when pumping through thedrill string up the inside of the riser the pressure inside the riser atseabed is regulated to just below that of seawater at that depth(gradient 305) by lowering or adjusting the air/liquid level 210 insidethe riser tube 201.

The formation soils being removed by the drill bit are pumped up tosurface by the pump system 202. As the hole deepens the riser andstructural conductor is lowered by help of the riser tensioning system501 until the structural conductor housing 234 is at an appropriateheight above seabed as shown in FIG. 2. In the process of removing soilsfrom the borehole the pressure 305 in the hole due to this operation canbe controlled by regulating level 210 of the liquid/air inside riser 201to lie between that of the pressure due to seawater gradient 302 and thesoil fracture gradient 301. As can be seen by FIG. 1, bringing thereturns from the well all the way back to the surface as in conventionaldrilling would not be possible. The hydrostatic pressure from thedrilling fluid gradient 304 would fracture the weak formation of thesoils, gradient 301 and the level 210 would not reach back to surfacebefore the returns would be lost to the shallow subsurface soils.

Further application of this system would include but not be limited toremoval of shallow seabed soils and particles on the ocean floor as inseabed mining. Seawater will flow into the riser tube and transport anysolids in suspension back up to the surface by the aid of the subseapump system 202.

1. A method for starting a surface hole in a seabed, comprising: using adrilling system comprising a structural conductor attached to a drillingriser suspended from a drilling platform whereby a lower end of thestructural conductor extends to the seabed, the drilling riser beingvented to atmosphere, a drilling string disposed within the drillingriser, the drilling riser comprising an outlet at a depth below thewater surface where the outlet is connected to a pumping system situatedon or above the seabed and below the water surface, the pumping systemhaving a return conduit running to the surface, the drilling systemfurther comprising a pressure control system configured for controllingpressure within the lower end of the structural conductor by operationof the pumping system to control fluid level in the riser; drilling afirst section of a surface hole in a seabed while operating the pumpingsystem so as to maintain a fluid level in the riser corresponding with apressure within the lower end of the structural conductor equal or lowerthan the higher of sea level pressure and soil fracture pressure presentat the lower end of the structural conductor, while pumping fluids andcuttings from the first section of the surface hole through thestructural conductor into the drilling riser and to the surface via theoutlet in the riser and the pumping system; and lowering the structuralconductor into the first section of the surface hole.
 2. The method ofclaim 1, further comprising: drilling a second section of the surfacehole in the seabed and while drilling, operating the pumping system soas to maintain the fluid level in the riser corresponding with apressure within the lower end of the structural conductor equal or lowerthan the soil fracture pressure at the lower end of the structuralconductor, while pumping fluids and cuttings from the second section ofthe surface hole through the structural conductor into the drillingriser and to the surface via the outlet in the riser and the pumpingsystem.
 3. A method for starting a surface hole in a seabed, comprising:using a system comprising a structural conductor attached to a drillingriser suspended from a drilling platform whereby a lower end of thestructural conductor extends to the seabed, the drilling riser beingvented to atmosphere, a drilling string disposed within the drillingriser, the drilling riser comprising an outlet at a depth below thewater surface where the outlet is connected to a pumping system situatedon or above the seabed and below the water surface, the pumping systemhaving a return conduit running to the surface, the system furthercomprising a pressure control system configured for controlling pressurewithin the lower end of the structural conductor by operation of thepumping system to control fluid level in the riser; drilling a firstsection of a surface hole in a seabed while operating the pumping systemso as to maintain a fluid level in the riser corresponding with apressure within the lower end of the structural conductor equal or lowerthan the higher of sea level pressure and soil fracture pressure presentat the lower end of the structural conductor, while pumping fluids andcuttings from the first section of the surface hole through thestructural conductor into the drilling riser and to the surface via theoutlet in the riser and the pumping system; lowering the structuralconductor into the first section of the surface hole; and drilling asecond section of the surface hole in the seabed and while drilling,operating the pumping system so as to maintain the fluid level in theriser corresponding with a pressure within the lower end of thestructural conductor equal or lower than the soil fracture pressure atthe lower end of the structural conductor, while pumping fluids andcuttings from the second section of the surface hole through thestructural conductor into the drilling riser and to the surface via theoutlet in the riser and the pumping system.
 4. The method of claim 1,said system further comprising said pumping system with said returnconduit adapted to be launched and run from a separate tender supportvessel (TSV) situated near the drilling platform, said pumping saidfluids and cuttings to the surface comprising pumping said fluids andcuttings to the TSV.
 5. The method of claim 1, said system furthercomprising said drilling platform comprising one from among the group ofplatforms consisting of a drilling vessel, a mobile offshore drillingunit (MODU), an anchored production platform, a SPARS or Bouy form, adeep-draft floater, an articulated steel tower, a floating drilling andproduction vessel (FDP), and a platform fixed to seabed with tensionlegs (TLP).
 6. The method of claim 1, said system further comprising thereturn conduit being connected to the drilling platform.
 7. The methodof claim 1, the angle of the surface hole being deviated from the angleof the riser.
 8. The method of claim 1, said system further comprisingsaid drilling riser connected to a conductor housing via a pinconnector, said conductor housing connected to said structuralconductor.
 9. The method of claim 2, said lowering the structuralconductor comprising concurrently extending the drilling riser so as tomaintain the structural conductor being attached to the drilling riser.10. A method for starting a surface hole in a seabed, comprising: usinga system comprising a structural conductor attached to a drilling risersuspended from a drilling platform whereby a lower end of the structuralconductor extends to the seabed, the drilling riser being vented toatmosphere, a drilling string disposed within the drilling riser, thedrilling riser comprising an outlet at a depth below the water surfacewhere the outlet is connected to a pumping system situated on or abovethe seabed and below the water surface, the pumping system having areturn conduit running to the surface, the system further comprising apressure control system configured for controlling pressure within thelower end of the structural conductor by operation of the pumping systemto control fluid level in the riser; drilling a first section of asurface hole in a seabed while operating the pumping system so as tomaintain a fluid level in the riser corresponding with a pressure withinthe lower end of the structural conductor equal or lower than the higherof sea level pressure and soil fracture pressure present at the lowerend of the structural conductor, while pumping fluids and cuttings fromthe first section of the surface hole through the structural conductorinto the drilling riser and to the surface via the outlet in the riserand the pumping system; lowering the structural conductor into the firstsection of the surface hole and concurrently extending the drillingriser so as to maintain the structural conductor being attached to thedrilling riser; and drilling a second section of the surface hole in theseabed and while drilling, operating the pumping system so as tomaintain the fluid level in the riser corresponding with a pressurewithin the lower end of the structural conductor equal or lower than thesoil fracture pressure at the lower end of the structural conductor,while pumping fluids and cuttings from the second section of the surfacehole through the structural conductor into the drilling riser and to thesurface via the outlet in the riser and the pumping system.